1. Field of the Invention
The present invention relates to an RS-485 interface, and more particularly, to a circuit for connecting one master node to a large number of slave nodes in a communication network management system.
2. Description of the Related Art
An RS-485 interface is a 2-wire differential drive transmission system. When a supervisory control system of a communication network (or transmission network) is designed using an RS-485 interface, a multidrop connection circuit having a master node and slave nodes is widely used. In communication between the master node and the slave nodes, there is used a polling technique by the master node using a unique identification (ID) number. In an earlier RS-485 multidrop connection circuit, the number (fan-out) of slave nodes which can be connected to one RS-485 common line is 32. The master node sequentially transmits/receives request and response data with the 32 slave nodes.
The master node and the slave nodes transmit/receive data by setting an enable signal to a logic "HIGH (1)" or logic "LOW (0)". One master node is connected to 32 slave nodes at a maximum through an RS-485 common line. The master node has a driver and a receiver. Each of the slave nodes also has a receiver and a driver. The communication between the master node and the slave nodes is done on the basis of the master node. In more detail, the master node sets an enable signal to a logic "LOW" level and transmits data by using an ID number of a slave node which it is to communicate with. If the data has been transmitted, the master node sets the enable signal to a logic "HIGH" level to maintain its drive at a high impedance state. The respective slave nodes receive the data from the master node. A slave node receiving request data corresponding to its own ID number sets its enable signal to a logic "LOW" level and transmits response data to the master node. If the response data has been transmitted, the corresponding slave node sets its enable signal to a logic "HIGH" level to maintain its drive at a high impedance state. Thus the master node sequentially communicates with all of the slave nodes.
However, since the earlier RS-485 multidrop connection circuit has a maximum of 32 slave nodes connected to one RS-485 common line, it is impossible to construct a supervisory control network consisting of hundreds or thousands of nodes. Therefore, it is necessary to expand the fan-out.
The following patents each discloses features in common with the present invention: U.S. Pat. No. 5,305,215 to Brekkestran et al., entitled Expandable, Mobile, Modular Microcomputer System For AN Off-Road Vehicle, U.S. Pat. No. 4,882,702 to Struger et al., entitled Programmable Controller With I/O Expansion Module Located In One Of I/O Module Positions For Communication With Outside I/O Modules, U.S. Pat. No. 5,060,134 to Hunninghaus, entitled Action Direction Port Expansion Circuit And System, U.S. Pat. No. 5,014,236 to Pogorzelski et al., entitled Input/Output Bus Expansion Interface, U.S. Pat. No. 5,802,327 to Hawley et al., entitled Device For SCSI Expansion, U.S. Pat. No. 5,805,833 to Verdun, entitled Method And Apparatus For Replicating Peripheral Device Ports IN AN Expansion Unit, U.S. Pat. No. 5,603,051 to Ezzet, entitled Input/Output Processor With A Local Memory providing Shared Resources For A Plurality Of Input/Output Interfaces On AN I/O Bus, U.S. Pat. No. 5,684,966 to Gafford et al., entitled Method For Operating A Repeater For Distributed Arbitration Digital Data Buses, U.S. Pat. No. 4,947,386 to Preschutti, entitled Fixed Gain Fixed Loss Amplification System, U.S. Pat. No. 5,455,700 to Thompson et al., entitled Regenerative Communication Channel Extender, and U.S. Pat. No. 5,673,258 to Helbig et al., entitled Method And Apparatus For The Enlargement Of The Reach Of The Transmission Channel Between Functional Groups Of ANISDN-User Interface.